Abstracts

RATIONALE: Alterations in the metabolism of GABA and its synthesizing enzyme glutamate acid decarboxylase (GAD) have been proposed to play a critical role in temporal lobe epilepsy (TLE). Recent studies describing alterations in the hippocampal expression of the two GAD isoforms in models for TLE prompted us to investigate possible seizure-induced alterations in the expression of GAD isoforms in the piriform cortex (PC), i. e. a limbic brain region which has gained growing interest as a possible key structure in the generation and propagation of limbic and secondary generalized convulsive seizures. Furthermore, our investigations included the hippocampus, the amygdala, and the substantia nigra pars reticulata (SNr), i.e. regions thought to be involved in limbic seizure activity.
METHODS: A self-sustained status epilepticus (SE) was induced in rats by i.p. administration of 10 mg/ kg kainate (KA) or 30 mg/ kg pilocarpine (PILO) plus lithium. Duration of the SE was limited to 90 minutes with diazepam, and brain regions were examined 8 h after injection of the convulsants or vehicle. Nonradioactive in situ hybridization (ISH) using digoxigenin -labelled probes against the mRNAs for GAD65 and GAD67 was performed . The expression of the GAD mRNAs was compared with immunhistochemical labeling of neurons for GABA and GAD.
RESULTS: In the anterior PC, the KA- or PILO-induced SE resulted in a decreased number of neurons positively labelled for GAD65 and GAD67 mRNA in the ventral endopiriform nucleus (VEN), whereas no alterations were seen in PC layer II. In contrast, a higher GAD mRNA expression was seen in layer II of the central PC, while GAD65 and GAD67 labelled neurons were reduced in layer III and VEN. In the posterior PC, results were similar to the anterior PC. Whereas decreases in number of GABAergic neurons in layer III and VEN/EN of the PC were also seen with immunohistochemical labeling of neurons in the PC, the higher activity of layer II neurons determined by ISH in the central PC was not detectable with GAD or GABA immunohistochemistry. In the hippocampus, an upregulation of GAD67 mRNA was determined in granule cells in KA and to a higher extend in PILO treated rats. In the amygdala and SNr, a marked loss of GABAergic neurons was seen in both models.
CONCLUSIONS: The present data indicate that GABAergic cells in the layers of the PC are affected differently by SE. In layer II of the central PC an upregulation of GAD in GABAergic neurons might contribute to counteract hyperexcitability, while in the deeper layers the majority of interneurons is already degenerated shortly after SE. These findings provide further evidence for the complexity of changes in GABA metabolism in the PC during limbic seizure activity.
Support: Deutsche Forschungsgemeinschaft